In general, various machines having one or more operating sections each of which is driven by a servomotor, e.g., industrial robots, NC machine tools, etc., are provided with servo systems for respective axes of the machine, and the servo systems responding to a movement command supplied from a numerical control unit. Typically, each of the servo systems is arranged to periodically distribute pulses in response to a movement command. Namely, the servo system distributes pulses whose number corresponds to a target movement amount of a respective servomotor with one pulse distribution time period in such a manner that a moving speed of the servomotor is uniformalized during that period, to thereby drive the servomotors for the individual axes and hence the various machine operating sections to their target positions.
During a machine operation, each of the servo systems checks the error amount between a target moving position and an actual moving position of an associated servomotor periodically, detects the excess of the actual position error, and regards it as an occurrence of an abnormality, e.g., overload of the servomotor, interference between the machine operating section concerned and a foreign object or the like, so as to stop the drive of the associated servomotor, whereby the respective servo systems and the servomotors are protected.
In case that a distal end of a complicated mechanism is moved along a certain path at a constant speed, there often occurs a change in a target moving amount and in a target moving speed of a respective axis servomotor. On this occasion, a response delay in a servo system increases as the target moving speed is increased. In this respect, conventionally, a permissible position error is fixedly set in dependence on the maximum rotational rate of an associated servomotor, so as to prevent an increased actual position error due to a normal response delay from being erroneously determined as occurrence of an abnormality even if the maximum response delay appears in the servo system when the target moving speed corresponding to the maximum rotational rate is specified.
However, according to the aforesaid prior art, even if a certain movable section of a machine interferes with a foreign object during its low-speed movement, for instance, an occurrence of an abnormality cannot be detected so long as the actual position error does not exceeds the permissible value which is fixedly set in dependence on the maximum motor rotational rate, so that the servomotor associated with the machine movable section is permitted to be continually driven. As a consequence, an excessive electric current flows in the servomotor so as to permit the same motor to produce an excessive output torque, whereby the servomotor and the movable mechanism can be damaged.